Quick answer
Run an inverter at 80% or less of its continuous rating — a 2,000W unit is honestly a 1,600W unit. Above that, three things bite: heat shortens component life, the surge headroom you need for motor starts disappears, and the wiring/fusing around the inverter (sized to its full rating) runs closer to its limits than your loads require. The exceptions are brief peaks — starting a fridge compressor for a second at 1,900W on a 2,000W inverter is what surge ratings are for; running a 1,900W load for an hour is not. The math and the exceptions are below.
How to read this page: this is an engineering-practice guide, not a test report — we have not bench-tested any inverter. The 80% rule and the derating factors below are standard practice drawn from how manufacturers rate their units (continuous vs surge) and how installers size systems; where a figure is an editorial planning rule rather than a code requirement, it is labeled as such. Our how we recommend page covers the site’s standards.
Continuous, surge, and the number that lies
An inverter’s spec sheet lists two wattages that get confused constantly:
- Continuous rating — what the unit can sustain indefinitely at 25°C ambient. This is the number the box shouts.
- Surge/peak rating (typically 2× continuous for seconds) — what it can deliver while a motor starts.
The rating that lies is continuous at what temperature: sustained output drops as the inverter heats up (see below), so the practical continuous number in a hot equipment bay is lower than the brochure. The honest sizing chain: your simultaneous running load ≤ 80% of continuous; your worst motor start ≤ the surge rating.
The worked math: sizing with the 80% rule
The load list. A cabin runs: refrigerator 200W (running), LED lights 100W, laptop + phone 100W, and occasionally a microwave at 1,500W input — but the microwave runs alone (nothing else starts simultaneously by house rule).
- Simultaneous running load (no microwave): 400W → any quality 600W+ inverter covers it at 80% (400 ÷ 0.8 = 500W minimum).
- With the microwave: 1,900W → needs a 2,400W-class inverter at the 80% rule (1,900 ÷ 0.8 = 2,375W). A “2,000W” inverter technically fits 1,900W — and that’s exactly the sustained-near-max trap this guide exists to prevent.
- Surge check: the refrigerator’s compressor start (~600–1,200W for a split second) on top of 300W of lights: ~1,500W — comfortably inside a 2,400W unit’s ~4,800W surge rating.
Run your own list through the inverter sizing calculator, then apply the 80% rule to its answer.
The three derating factors
1. Heat. Electronics derate as they warm — a unit that sustains 2,000W at 25°C sustains less at 45°C inside a closed cabinet in July. Editorial planning rule: keep continuous loads at 80% and give the inverter ventilation (its manual’s clearances are real, not decorative). In hot-climate enclosed installs, be more conservative still.
2. Altitude. Thinner air cools worse; manufacturers commonly publish reduced continuous output above roughly 2,000 m / 6,500 ft (check your unit’s manual for the specific derate — it varies by maker and is a per-manufacturer spec, not a universal number). Mountain installs should size with that reduced figure.
3. Battery-side reality. At 12V, a 2,000W load at 90% inverter efficiency draws ~185A — a current level where cable length, lugs, and fusing start mattering as much as the inverter (the full chart is our battery cable size guide). This is a big part of why bigger loads push systems to 24V/48V: the voltage comparison halves and quarters that current.
What actually happens at sustained 100%
Nothing dramatic at first — that’s the trap. The unit runs, maybe for months. But its cooling system runs flat-out, components live at their thermal limits, and the surge headroom that would have absorbed a compressor start is spent. Community failure reports pattern-match: inverters that “ran fine at max” die during the July week everything runs at once, or trip offline exactly when the well pump starts. You paid for surge capability; sustained max loading is how you never get to use it.
Loading quick-reference
| Your simultaneous load | Buy an inverter rated (continuous) | Why |
|---|---|---|
| 400W | 500–600W+ | 80% rule with margin |
| 800W | 1,000–1,200W | plus motor-start headroom |
| 1,600W | 2,000–2,400W | the “2,000W inverter” trap zone — buy 2,400 |
| 2,400W+ | 3,000W+ (and consider 24/48V) | battery-side current becomes the story |
Frequently Asked Questions
Is the 80% rule a code requirement?
Can I exceed 80% for short periods?
Does derating apply to power stations too?
How do I find my inverter's altitude derating?
Next logical reads
Inverter sizing calculator Pure sine vs modified sine Battery cable size chart Inverter cost guide